1. Field of the Invention
The present invention relates to a power converter comprising a power conversion unit having an n-phase alternating current output (n is an integer number with n≧2) connected to a direct current power source, and smoothing means provided between the direct current power source and the power conversion unit, and a power unit using the same.
2. Related Background Art
In recent years, a system tied-distributed power source such as a photovoltaic power generating system has been in widespread use.
In such a power unit, the direct current power outputted from the direct current power source is inputted to an inverter serving as the power conversion unit, and is outputted to a system serving as a load. Further, the input portion of the inverter is connected to smoothing means, thereby stabilizing the voltage and the current of the direct current power source. In general, this smoothing means has a smoothing capacitor connected in parallel to the input portion of the inverter.
Meanwhile, since the instantaneous power of a single phase inverter fluctuates at the frequency two times an output alternating current, there is arisen a power fluctuation two times an output frequency between the direct current power and the output power from the direct current power source, and it is necessary for the smoothing capacitor absorbing a relatively low frequency fluctuation to have a large capacity. Hence, as for the smoothing capacitor, an aluminum electrolytic capacitor is selected, which is characterized in that the size and mass per electric capacity are small and the cost is low (for example, refer to Japanese Patent Application Laid-Open No. H01-107661).
Further, as for the inverter making a three-phase output, it is known that the electrolytic capacity of the smoothing capacitor becomes small (for example, refer to Japanese Patent Application Laid-Open No. S58-33976).
However, if the electrolytic capacity of the smoothing capacitor used as the smoothing means becomes even smaller, it is possible to realize additional downsizing, weight saving and lower cost and, thus, the smoothing means is desired to be smaller in capacity.
Further, it is generally known that the aluminum electrolytic capacitor is extremely short-lived due to increase in ambient temperature and self-heating. When the possibility of giving a long life to the power unit is considered, a question which arises is a lifetime of the aluminum electrolytic capacitor. Use of a solid capacitor, which is said to deliver a life longer than a film capacitor, a laminated ceramic capacitor and the like is conceivable, but an increase in the size and mass as well as cost up come into question.
On the other hand, as for the inverter making the three-phase output, just simply making the three-phase output does not make electric capacity sufficiently small under the existing circumstances.
Furthermore, not only for the smoothing capacitor, but also for other smoothing means such as a smoothing reactor, an active power filter and the like, which stabilize the voltage and current of the direct current power source, it is similarly desired to make the capacity small.